JP2003-172310A published by the Japan Patent Office proposes a pilot type change-over valve for controlling a flow of pressurized oil by moving a spool with a pilot pressure and changing over passages formed in a spool housing.
In this type of change-over valve, the spool is assembled into a spool hole, which is formed in the spool housing so as to be free to slide in a state where an end surface of the spool faces a pilot chamber.
When a pilot pressure is applied to the pilot chamber while the air mixed into working oil remains in the pilot chamber as bubbles, a target pilot pressure cannot be precisely applied to the spool due to shrinkage of the bubbles. Thus, the prior art proposes to provide a small hole for discharging the air in the pilot chamber in accordance with displacement of the spool.